The present invention relates to an interface circuit, particularly for use in motor vehicles between a direct current voltage source such as a battery and a circuit for the pulsed driving of a load such as an electric motor, in parallel with which a capacitor with a high capacitance is connected.
In circuits that are used for the pulsed driving of large loads, for example, with PWM (pulse-width modulation), for example, in order to keep the voltages at the terminals of the load constant with variations in the supply voltage, or to render the speed of operation of an electric motor variable, it is necessary to use very high-capacitance filtering capacitors, typically of the electrolytic type, in order to reduce electromagnetic emissions in accordance with the regulations in force.
In the automotive field, where a battery supply voltage of about 12V is typically available, the currents which flow in the supply wires of the electromechanical actuators typically have high intensities and this involves an increase in the capacitance of the above-mentioned capacitors. These capacitors have to have a very low series resistance (ESR) and to be able to withstand a ripple current which increases with increases in the load.
During the driving of such loads, the so-called “load dump” condition may also arise; this typically happens when the generator of the motor vehicle is disconnected from the battery suddenly, for example, owing to a vibration or a shock, whilst it is operating with a large current in order to charge the battery. The result of such a disconnection is a rapid decrease in the current flowing through the generator and a very high pulsed voltage of the order of hundreds of volts is consequently generated, in accordance with the well-known equation V=LdI/dt.
In selecting the dimensions for the electronic components of the interface circuitry for a load for automotive use it is therefore necessary to take account of the voltage pulse which may be produced in a “load dump” condition. However, various problems arise.
In the first place, there is a problem with regard to electrolytic capacitors since their size increases considerably with increases in their nominal working voltage, for a given capacitance value.
Moreover, if a MOSFET is used for driving an inductive load, in order to reduce losses in conduction it would be necessary to reduce the drain-source resistance RDSon but this conflicts with the need to have a high breakdown voltage VDS.
In circuits which comprise at least one high-capacitance capacitor, there is also the problem of limiting their initial load current or “inrush current”. During the initial transitory stage of the charging of a discharged capacitor having a capacitance, for example, of a few hundred microF, the current value is limited in practice purely by the resistance of the wiring and/or of the connectors as well as by the output limitations of the voltage source available, for example, 400A in the case of a motor-vehicle battery. It is therefore necessary to select correctly the dimensions of the wiring and the connectors as well as of all of the electronic components which are in series with the line through which the current is supplied to the capacitor/s in question, such as reversed-polarity protection diodes, etc.
An interface circuit of the type specified above for protecting a load against an excessive voltage pulse is known from European patent application EP-A-0 708 515. This known circuit comprises a channel-depletion MOSFET transistor and does not permit controlled limitation of the inrush current. The breakdown voltage VDS must be greater than the “load dump” voltage and it is therefore essential to use a protection diode in parallel between the drain of the MOSFET and the earth.
With reference to FIG. 1, a known solution which enables the inrush current to be limited is constituted by a circuit comprising a resistor R, through which the filtering capacitor C is charged, and a relay (or a MOSFET) M which is in parallel with this resistor and is closed when the voltage at the terminals of the capacitor C has reached a predetermined value. This solution does not overcome the problem of maintaining low absorption when the vehicle is in the inoperative (standby) stage if the capacitor C has leakages; this condition may arise in particular with electrolytic capacitors and could lead to permanent failure of the circuit if the leakages were to exceed the current permitted by the resistor R.
Another known solution for overcoming the problem of limiting the inrush current is that of inserting a MOSFET device M in series with the filtering capacitor C (FIG. 2). The filtering efficiency of the capacitor/s is reduced since it/they have the resistance RDSon in series with their own intrinsic resistance.